In order to complete an optical circuit by interconnecting optical circuit elements with optical fibers, it is necessary to accurately align the cores of the optical fibers with the optical axes of the optical circuit elements. To facilitate the alignment with the optical axes, optical fiber guide structures have been employed. For example, at the terminals of optical integrated circuits such as integrated light switches or optical couplers, a multicore optical connector is used to connect plural optical fibers in a batch according to plural optical waveguides. At this time, at the junctions of the optical waveguides and the optical fibers, connection loss occurs. As the degree of alignment of the optical waveguides with the optical fibers is enhanced, the connection loss decreases advantageously. Accordingly, multicore optical connectors capable of accurately setting the positions at which the optical fibers are fixed have been required.
A conventional multicore optical connector comprises a semiconductor substrate made of silicon (Si), gallium arsenide (GaAs), or other material. Grooves of V-shaped cross section are formed in the substrate by anisotropic etching. FIG. 10 is a cross sectional view of an optical fiber guide structure 28 having such V-grooves. A plurality of V-grooves 27 are formed in the top surface of a substrate 21. Optical fibers 29 are held in these grooves. Under this condition, the connector is used.
Also, a substrate similarly provided with V-grooves is used to interconnect optical elements by optical fibers. In particular, the optical elements such as a light source, polarizers, and collimators are arranged on the substrate and aligned to a given optical axis. The optical fibers are held in these V-grooves. Thus, the optical elements can be optically interconnected.
The V-grooves 27 in the optical fiber guide structure 28 are formed in the manner described now. The top surface of the silicon substrate 21 is of (100) orientation. This top surface is coated with a masking material such as silicon oxide (SiO.sub.2), platinum (Pt), or titanium (Ti). Windows of a given width are formed photolithographically such that they are regularly spaced from each other. Then, an etching operation is carried out within an anisotropic etchant such as aqueous solution of potassium hydroxide, aqueous solution of ethylenediamine-pyrocatechol, or aqueous solution of hydrozine. As a result, the V-grooves 27 are formed.
In this V-grooved structure for alignment of optical fibers, each optical fiber 29 is held by two inclined surfaces 27a and 27b of each V-groove 27. Therefore, even if the dimensional accuracy of the V-grooves 27 is not high and the widths 27c of the windows differ from each other considerably, the optical axes 29a of the optical fibers 29 hardly move horizontally. However, if the widths 27c of the windows are too great, the positions of the optical axes 29a of the fibers 29 are lowered. Conversely, if the widths 27c of the windows are too small, the positions of the optical axes 29a of the fibers 29 are raised. In this way, with respect to the vertical direction, the positions of the optical axes 29a of the optical fibers 29 are varied greatly.
It is easy to spread the terminals of optical waveguides horizontally, in which case the horizontal accuracy of multicore connectors does not present serious problems. However, it is difficult to spread the terminals of optical waveguides vertically. It is required that the optical axes of multicore optical connectors be placed in position at high accuracy, especially vertically.
For optical elements not assuming the form of an optical waveguide such-as light sources, polarizers, and collimators, it cannot be expected that the effect of misalignment of optical axes is reduced by modifying the shape of the terminals of the optical waveguides as mentioned above. Hence, the optical axes are required to be placed in position at high accuracy both horizontally and vertically.
However, where the V-shaped grooves are formed by anisotropic etching, it is inevitable that the widths of the windows in the V-grooves differ from each other to some extent because of the presence of residual stress and crystal defects in the substrate and because the reaction rate differs from location to location within,the etchant. Consequently, where the aforementioned multicore optical connectors are used or where optical elements aligned with given optical axes on a substrate are interconnected by optical fibers, the accuracy at which the optical axes are placed in position can be improved only up to .+-.0.5 .mu.m. For this reason, the prior art optical fiber guide structure suffers from large connection loss due to vertical shifts of optical fibers. Also, much labor is required to place the optical fiber guide structure in an optimum position where the connection loss is reduced to a minimum.